While searching for interesting items to add to my Global Sets, I've stumbled onto some Portal Playtest Cards from a couple playtesters.
I plan to keep the cards that fit my Global Sets, and sell off the rest to recoup my costs, but before any of them head off to new homes, I wanted to document them here for future reference. I've also researched some additional info on these cards from other sources, and will include that here also. Maybe someday we'll know what the complete set looks like.

For anyone who isn't aware, Playtest cards are what the Research & Development team uses to test how new cards interact with each other during set development. This helps to discover problems with the cards, and also helps to make the set balanced. Playtest cards are the first version of a card, before it gets printed and becomes a regular real card. Some playtest cards never make it beyond this testing stage.
Changes are made to the cards throughout playtesting, and it's normal for a set to have a few generations of playtesting.
Playtest cards have been made a few different ways over the years. During the mid 1990's, Wizards of the Coast Research & Development found it convenient to print the new card info onto stickers, and adhere those onto regular Magic cards for playtesting. They used this process for many many years, and may still be using it today. Playtest cards were created differently prior to this. The first generation of Portal Playtest cards are the only Playtest cards that I've seen with half sized stickers. I suspect that these are the very first attempt at using stickers for Playtest cards.

During the time these cards were made, official WotC checklists were organized in Black, Blue, Green, Red, White, (BUGRW) order. This is alphabetical order, which is the way the computer organized them.
I will list the cards in that order here, separated by generation.
(White, Blue, Black, Red, Green (WUBRG) order was used for Mirage & Visions, and then on 1998 and later Magic sets. This is clockwise order of the mana symbols on the card back, starting at the top.)

Now that we're through the introductions, On to the good stuff...

Each card has a 4 digit identification code.
The first digit indicates the card rarity.
c=Common
n=Uncommon
r=Rare
The second digit indicates the card color.
b=Black
u=Blue
g=Green
r=Red
w=White
The last two digits indicate the card number. Each color and each rarity is numbered separately.
For example: Commons are numbered 1 through 17 in each color, while Uncommons are numbered 1 through 13 in each color
(Note: Mark Rosewater has mentioned that in general, sets frequently had 18 Commons in each color. This means that we may also be missing the 18th card in each color here, but I won't add it to the lists until we have some evidence that it exists.)

In addition to organizing these Playtest cards by generation, I will organize them by color, rarity, and card number.
This will allow us to see how many Playtest cards we are missing. Hopefully more will be found in the future, and we can fill in some of the blank spaces.
I used a bright light behind each card, to look through the card and see what regular card it was. I will also include here, what regular card the Playtest sticker has been attached to. This will provide some insight into when the playtesting occurred for each generation.

While a new set is being designed, it doesn't yet have an official name. Instead, it has a Codename.
Codenames began with Alliances as an internal office prank. Someone discovered that if you gave a computer folder the same name as a sound file, the computer would play the sound file whenever the folder was opened. Alliances was given the codename Quack, and whenever anyone in the office opened the computer files to work on the set, the whole office would hear the audible Quack sound file.
The first generation of Portal playtesting was created before Alliances or codenames. A few cards from the first generation of Portal playtesting were added to Alliances playtesting and actually got printed there before / instead of in Portal.
Portal received its codename for the second generation of playtesting. The Codename for Portal was Harvey.
Harvey is abbreviated as HA on the second generation and third generation Playtest cards. It does not appear on the first generation Playtest cards.
During the first generation of Portal Playtesting, members of the design team were confused about the expectations behind the Portal concept. The Codename Harvey comes from a 1944 play (1950 movie starring James Stewart), in which Harvey is a big invisible rabbit, and no one knows what he is.

The first and second generations of playtesting were done internally by the Research & Development team, as is typical.
However, because this set was designed for teaching beginners, they really needed some feedback from people who had never played the game.
The third generation of playtesting was advertised locally in Renton Washington where WotC was located. Kids who had never played could sign up, and be provided with 5 prebuilt decks and some instructions. These 5 decks were assembled by R&D, and based on the Portal tip card deck suggestions. For Example: Everyone received a Blue & White "Air Power" deck, but these "Air Power" decks were not identical to each other. Every "Air Power" deck was a little different, containing some different Blue & White cards. It was the same way for the other four decks. No one received a full set of Portal Playtest cards, they only got their 5 decks. This means that if you were playing your "Air Power" deck against someone else's "Air Power" deck, both players would see some new cards that they weren't familiar with.

A note on rarity: All Playtest cards are pretty rare. Most of them are lost to the years, having been thrown away or destroyed long ago.
One of the playtesters told me that they believed at least 100 kids received 5 decks each. From what I've seen, I only have one copy of most of these cards. There are also several cards that I have two copies of. There are a few cards that I have several copies of, sometimes more than 4 copies.

1st Generation Portal Playtest cards use half sized stickers. They reuse the original cards name and casting cost, so it was important that the Playtest sticker be attached to the correct card. Next to the identification code, is the name of the card that the sticker should be attached to. These Playtest stickers can change the card type, rules text, & power/toughness. They introduce the sword/shield icons for power/toughness. These 1st Generation cards do not indicate a set Codename. Most of the 1st Generation Playtest stickers are attached to English 4th Edition cards, however, there are also some English Ice Age cards used, so this generation occurred sometime after June 1995.
(Note: Bill Rose was the lead designer for Portal. He began officially working at WotC in October 1995, but he was also an Alpha playtester and had worked on Magic since the beginning, so he could have started work on this set before moving to Washington state.)
I have not seen any Uncommons or Rares from the 1st Generation, and they might not exist.

2nd Generation Portal Playtest cards use full size stickers, like most subsequent playtest cards. This simplified the process, because Playtest stickers could now be placed onto any card, instead of having to search for a specific correct card. The Playtest stickers now control all features of the card, including name and casting cost. These 2nd Generation cards have "HA", an abbreviation of the set Codename, located where the expansion symbol would be. The stickers were placed onto cards of a matching color. (blue stickers on blue cards, etc.) Most of the 2nd Generation Playtest stickers are attached to English 4th Edition cards, but there's also Revised. Based on the info from 1st Generation and 3rd Generation, playtesting for the 2nd generation probably occurred during early 1996. It may appear that all the black cards were put on Erg Raiders, all the blue cards were put on Erosion, all the green cards were put on Land Leeches, and all the red cards were put on Eternal Warrior, but I think this is just a coincidence. They just happened to have a lot of copies of these particular cards around, because no one wanted to play with these cards. Cards that no one wanted, were used for applying Playtest stickers.
I have not seen any Uncommons or Rares from the 2nd Generation, and they might not exist.
After the 1st Generation of Playtesting, some of the cards might have been modified or replaced with different cards for the 2nd Generation. For example, Cg03 was Pradesh Gypsies during the 1st Generation, and cg03 may or may not have been Pradesh Gypsies during the 2nd Generation. There isn't enough information yet, to know which cards were modified or replaced for the 2nd Generation.

3rd Generation Portal Playtest cards use full size stickers, like most subsequent Playtest cards. These 3rd Generation cards have "HA", an abbreviation of the set Codename, located at the bottom of the card, just before the Identification Number. They have "v2.0" where the expansion symbol would be. The stickers were placed onto cards of a matching color. (blue stickers on blue cards, etc.) Most of the 3rd Generation Playtest stickers are attached to Fallen Empires & German Ice Age cards, but there's also English Ice Age, English Alliances, and English Mirage, so this generation occurred sometime after September 1996. This generation has a lot more variety in what cards the stickers were applied to. Again, undesirable cards were used for applying Playtest stickers. It's unusual to find Playtest stickers covering good cards. The best things here are probably Hymn to Tourach & Goblin Grenade.
The 3rd Generation was the public portion of Portal playtesting. Leftover cards from the 1st and 2nd generations were also included in 3rd Generation Playtesting. All of the cards seen in this article come from 3rd Generation playtesters.

A Note on Identification Numbering: How many different Playtest cards were actually made?
We know that each color was numbered separately because we have numerous examples within each generation of the same number being used on different colors. For example: 1st Generation has a number 5 card in Black, Green, Red, & White. 2nd Generation has a number 6 card in Blue & White. 3rd Generation has a number 5 Common card in Black, Blue, Green, & Red.
We know that each rarity was numbered separately because we have multiple examples within 3rd generation of the same number being used on different rarities. For example: 3rd Generation Black has a number 5 card in Common & Rare.
It's pretty safe to assume that the numbers start at 1 and go upward.
It's also pretty safe to assume that each color has an equal number of cards as the other colors, and they're divided up into rarity in the same proportions. The final finished product Portal set is that way, and it's typical design for having a balanced game.
The thing that we Don't know is how high the numbers should go.
1st, 2nd, & 3rd Generation all have Commons numbered up to 17. We haven't seen any Commons higher than 17.
We haven't seen any Uncommons or Rares from 1st or 2nd Generation, but 3rd Generation has Uncommons numbered up to 13, and Rares numbered up to 8.
Artists are given the Playtest card name and Identification Number when they create the card artwork. Some artists include this information on the back of the artwork, or in the margin. I've looked at a few different pieces of original art from Portal, and one piece did have an Identification Number, RW11. Apparently the Rares go up to at least 11.
Checking the math, I think that puts us pretty close to what should be the complete set.
17 Commons of each color = 85 cards
13 Uncommons of each color = 65 cards
11 Rares of each color = 55 cards
For a total of 205 cards (+ land & tip cards)
For comparison, Gatherer lists the finished product Portal at 195 cards (+ land & tip cards)

NOTE: Resurection has a misprint. It's missing part of it's Identification Number. We'll need to find the other White Uncommons before we can figure out what number Resurection was supposed to have.
V2.0 HA nw??----------Resurection-------------ENG Ice Age Kjeldoran Escort with spear artwork (not bow)--Breath of Life

This is what the land cards look like.
I'm not sure what generation these are from, they could be from any of the generations.
They're all on English 4th Edition cards, and every different artwork of English 4th land was used.
Interestingly, several of these Portal Playtest lands are on Alternate 4th Edition cards. None of the other Portal Playtest cards I've seen were on Alternate 4th Edition cards, only some of the lands. This tells us that the Wizards of the Coast Research & Development department did have some Alternate 4th Edition cards on hand.